Olefin metathesis is one of the most recent catalyzed reactions of hydrocarbons to emerge. Discovered in 1959 this reaction opened up a new and exciting field of hydrocarbon chemistry and provided chemical routes for the interconversion of light olefinic hydrocarbons, the backbone of today's petrochemical industry, and for the synthesis of high-purity olefins for the specialty chemicals market. The reaction is general for hydrocarbons containing olefinic bonds. The reaction was originally referred to as disproportionation. The more appropriate name "metathesis" was introduced in 1967 and is now commonly used.
Metathesis can be visualized as a net breaking and reformation of two olefinic carbon-carbon bonds. A generalized metathesis reaction can be represented as follows: ##STR1## Each of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7 and R.sub.8 is independently representative of hydrogen or a hydrocarbon group. A simple but commercially significant example of the reaction is the conversion of propylene to ethylene and normal butenes.
Olefin metathesis reactions are presently believed to proceed through a single-step metallocarbene scheme involving a metallocyclobutane intermediate as illustrated below: ##STR2## where M is W, Mo or Re catalytic sites. Support for the validity of this generally accepted mechanism has been provided by very detailed kinetic and mechanistic studies conducted by several groups of investigators. However, for predicting primary products of industrial metathesis applications, the simple "four-center" or "quasi-cyclobutane" concerted mechanism shown below is adequate and more direct: ##STR3##
As depicted in both schemes, the types and total number of bonds remain unchanged; as a result, metathesis reactions are essentially thermoneutral. Another characteristic of the metathesis reaction is that the integrity of the alkylidene moieties is retained during transformation.
Metathesis reactions of acyclic mono-olefins can be classified into two groups: (1) self-metathesis of a single olefin and (2) cross-metathesis of double-bond isomers or of two different olefins. In the first case, two primary metathesis products are produced, e.g., propylene yields ethylene and 2-butene. In the second case, sets of both self- and cross-metathesis products are obtained. For example, the metathesis of a 1-pentene/2-pentene mixture yields the following sets: ethylene/4-octene, propylene/3-heptene, 1-butene/2-hexene, and 2-butene/3-hexene. When ethylene is one of the reactants, alpha-olefins are produced as a consequence of "ethylene cleavage". For example, ethylene cleavage of 3-hexene yields 1-butene. Cross-metathesis of acyclic olefin/cyclic olefin mixtures yields diolefins. For example, ethylene cleavage of cyclic olefins provides a new route for the production of alpha, omega-diolefins as illustrated below: ##STR4##
The metathesis of other types of olefins and olefin mixtures, including diolefins, ring-substituted olefins (e.g., styrene), and functionally-substituted olefins produces products consistent with the above schemes. In theory, the number of olefin metathesis reactions is limited only by the number of compounds containing carbon-carbon double bonds. Metathesis reactions include, but are not limited to, the following:
(1) The metathesis of an acyclic mono- or polyene having at least three carbon atoms into other mono- or polyenes of both higher and lower number of carbon atoms; for example, the disproportionation of propylene yields ethylene and butenes; the disproportionation of 1,5-hexadiene yields ethylene and 1,5,9-decatriene;
(2) The reaction of an acyclic mono- or polyene having three or more carbon atoms and a different acyclic mono- or polyene having three or more carbon atoms to produce different acyclic olefins; for example, the reaction of propylene and isobutylene yields ethylene and isopentene;
(3) The reaction of ethylene and an internal acyclic mono- or polyene having four or more carbon atoms to produce other olefins having a lower number of carbon atoms than that of the acyclic mono- or polyenes; for example, the reaction of ethylene and 4-methylpentene-2 yields 3-methylbutene-1 and propylene;
(4) The reaction of ethylene or an acyclic mono- or polyene having three or more carbon atoms with a cyclic mono- or cyclic polyene to produce an acyclic polyene having a higher number of carbon atoms than that of any of the starting materials; for example, the reaction of cyclooctene and 2-butene yields 2,10-dodecadiene; the reaction of 1,5-cyclooctadiene and ethylene yields 1,5,9-decatriene;
(5) The reaction of one or more cyclic mono- or cyclic polyenes to produce a cyclic polyene having a higher number of carbon atoms than any of the starting materials; for example, the reaction of cyclooctene yields cyclohexadecadiene;
(6) The reaction conversion of an acyclic polyene having at least 7 carbon atoms and having at least 5 carbon atoms between any two double bonds to produce acyclic and cyclic mono- and polyenes having a lower number of carbon atoms than that of the feed; for example, the reaction of 1,7-octadiene yields cyclohexene and ethylene; or
(7) The reaction of one or more acyclic polyenes having at least three carbon atoms between any two double bonds to produce acyclic and cyclic mono- and polyenes generally having both a higher and lower number of carbon atoms than that of the feed material; for example, the reaction of 1,4-pentadiene yields 1,4-cyclohexadiene and ethylene.
In addition to olefinic reactants the metathesis reaction can successfully proceed with acetylenic reactants. For example the metathesis of 2-pentyne yields 2-butyne and 3-hexyne.
Metathesis catalysts include both homogeneous and heterogeneous catalysts. Based on current commercial activity the heterogeneous catalyst appears to have the greatest utility. Among the most effective metathesis catalysts are the oxides of molybdenum, tungsten and rhenium supported on a high surface area alumina or silica. Typical compositions and physical properties of three such metal oxide catalysts are given below in Table I.
TABLE I ______________________________________ Typical Compositions, Physical Properties and Reaction Temperatures of Three Common Heterogeneous Metathesis Catalysts CoO.MoO.sub.3.Al.sub.2 O.sub.3 WO.sub.3.SiO.sub.2 Re.sub.2 O.sub.7.Al.sub.2 O.sub.3 ______________________________________ Composition Wt % MoO.sub.3 11.0 WO.sub.3 6.8 Re.sub.2 O.sub.7 14 CoO 3.4 Al.sub.2 O.sub.3 85.6 &lt;0.1 86 SiO.sub.2 93.2 Physical Properties Surface Areas, 285 345 255 m.sup.2 /g Pore Volume, 0.58 0.98 0.37 cc/g Avg. Pore 82 114 58 Diameter, .ANG. Reaction Temp., 100-200 300-500 0-100 .degree.C. ______________________________________
Molybdenum oxide-alumina and cobalt molybdate-alumina catalysts are readily and commercially available for metathesis applications. These catalysts exhibit their best metathesis activity in the 100.degree.-200.degree. C. temperature range.
Tungsten oxide-silica is a commercially available catalyst developed specifically for metathesis by the Phillips Petroleum Company. This catalyst is best suited for metathesis in the 300.degree.-500.degree. C. temperature range. It is less susceptible to trace quantities of catalyst poisons in the feed stream than are the lower temperature alumina-based catalysts.
The rhenium oxide-alumina catalyst is active for metathesis at ambient conditions. It can be prepared in the laboratory in accordance with a variety of techniques including the impregnation of high surface area alumina with aqueous ammonium perrhenate solutions.
Other combinations of the oxides and the supports can be successfully employed.
For more information concerning metathesis the reader is advised to seek out and read the following references:
1. G. C. Bailey (1969) "Catalysis Reviews" Vol. 3, pages 37-60.
2. R. L. Banks (1981) "Specialist Periodical Reports", Vol. 4, pages 100-129.
3. R. L. Banks (1979) "Chemtech" Vol. 9, pages 494-500.
Persons of skill in the art of olefin (and acetylenic) metathesis seek to impove (i.e. increase) the conversion of reactants to products. The selection of catalyst is an important factor. Accordingly, it is one object of this invention to provide a metathesis process of improved conversion. It is also an object of this invention to provide catalysts of improved utility including increased metathesis conversion.
Broadly, it is an object of this invention to provide a novel composition of matter.
It is a further object of this invention to provide a novel metathesis process.
These objects and other objects and advantages of this invention will become apparent to persons of skill in the art of metathesis upon reading this disclosure and the appended claims.